The structure of interphase nuclei will continue to be investigated at two distinct interlocking levels. First, the final folding patterns of polytene chromosomes from Drosophila melanogaster salivary glands will be studied by a variety of new techniques especially tailored to probe the third dimension, these techniques are X-ray lithography, three-dimensional fluorescence microscopy using antibodies to nuclear proteins, DNA specific fluorescent dyes and computerized image reconstruction. Second, the fine details of the polytene chromosomes bands/interbands and the corresponding diploid interphase chromosome structures will be determined by making use of high-voltage EM, quantitative polarization microscopy, fluorescence microscopy, X-ray lithography and the scanning EM. These techniques have been modified so that three dimensions can be studied. Specific DNA sequences will be localized to portions of the band/interbands of the polytene chromosomes. A great aid to the determination of the above structure has been the development of computerized image reconstruction. These novel methods will be extended to three dimensions and possible refinement of the structure in the computer with known models. These general approaches will be applicable to the different complementary microscopic methods. The biochemistry related to the sequences and proteins that reside on the polytene ectopic fibers will be correlated with the structural approaches.